1. Field of the Invention
This invention relates in general to multiple disk drive array storage devices (DASD), and more particularly, to a DASD array and method wherein spindles in the array are synchronized.
2. Description of Related Art
Disk drives have long been popular mass storage devices. They provide a cost-effective answer to the problem of non-volatile data storage. Pressure is continually being exerted on digital data storage system designers to increase the storage capacity and data processing capabilities of such systems. Initially, the focus was on providing higher density disks. This involved various techniques for reducing the physical space which the recorded date occupied. Another technique for providing additional storage utilizes multiple disks. However, multiple disk systems create unique challenges of their own.
The basic structure of disk drives consists of a metal disk coated with magnetic material rotating under one or more read/write heads. A DASD is a multi-platen system where a number of the metal disks are arranged in a stack. Digital data is recorded on disks in the form of magnetic transitions spaced closely together on concentric circles called "tracks". Disk drives contain detectors for indicating when the magnetic head is positioned at the outermost track. A stepper motor (or servo-controlled linear motor) controls the head position by causing the head to step from track to track.
Once a track is selected, it is necessary to wait for the desired location to rotate into position under the head. Within each track, information is organized into segments called "sectors". A sector can consist of any number of bytes, limited only by the storage capacity of the track. The addressing of sectors has typically been a software function. Thus, each sector normally has been proceeded by an identifier block so that the sectors can be identified by the software.
New methods have been devised for recording data wherein the recording head locates and identifies data sectors without using data ID fields. Further information on the no-ID disks is disclosed in the co-pending and commonly assigned application Ser. No. 08/173,541 filed on Dec. 23, 1993 by Steve Hetzler and William Kabelac entitled "SECTOR ARCHITECTURE FOR FIXED BLOCK DISK DRIVE", which application is hereby incorporated by reference. Data sectors are identified using information obtained from electronic storage and from servo sectors which need not be adjacent to the data sectors. The tracks contain servo information and data, but not data sector ID information.
Nevertheless, both radial track density and linear density along a track have increased to the point that imperfections known as media defects in the magnetic recording layer pose a problem. At very high densities, these imperfections impact the recording of the digital data across several transitions. As can be appreciated, this problem is compounded in DASD devices.
In DASD arrays, a number of different spindles must be synchronized. In such systems, it is often desirable to synchronize the rotation of all disk drives in the system to one master index and maintaining a constant lock to this index. Synchronization becomes very difficult when one of the spindles has a large number of the above-mentioned sector defects. Once a defect in encountered on any of the disks, all of the data on the disks with defects will be shifted in time as compared to a disk without defects. Data is written on the good file and the write head must wait until the defects are passed over on the defective disk until an acceptable area for writing data is encountered. Thus, the data on the defective disk will be out of phase by that many sectors. The spindle with the defects tends to lag more and more behind the other spindles.
In some prior systems, spindle synchronization has been carried out by allocating a sufficient number of spare sectors on each track to compensate for bad sectors. However, the allocations of disk space as spare sectors require a large overhead of disk space. These disk systems would allocate certain sectors as spares at fixed intervals on the disk. The data which would have normally been written in the defective sector is then written in the spare. The other data would be written in phase with the data on the non-defective disk.
For example, ten spares could be allocated at the end of every cylinder. If five defective sectors were encountered in that zone, then the data corresponding to the defective sectors would be written in the first five spare sectors. After reaching the end of the spares, the head would come to the next cylinder or zone where the disks would again be in synchronization.
However, allocating disk surface for spares, which may or may not be needed, is wasteful and thereby reduces the effective capacity of the disks. In this type of system, statistical analysis is performed to determine the number of spares needed. However, in actual tests the unused spare sectors accounted for between 2.5% to 3% of the total disk space. Furthermore, there may be instances when the number of defects may exceed the number of spare sectors allocated for that particular zone. In this situation, a spare would have to be stolen from the next zone. This means that the data allocated to these additional spare sectors would be moved out an additional amount of time. This would also negatively affect the head skew thereby necessitating an additional revolution of the disks.
It can seen then that there is a need for a DASD array and method wherein spindles in the array are synchronized in speed of rotation and phase.
There is also a need for a disk drive system having a plurality of spindles wherein synchronization is provided without wasting storage capacity due to unused spare sectors.